So, by the time the Red Army reached the Vistula, there were about a million reasons why the Soviets just sat back and held their position while the Polish rebels just a few miles away tried, and failed after several months, to uproot the Nazis from their capital. The Russians were tired, dis-organised, overstretched, and most importantly, Stalin did not give a mad-ass fuck, since helping out potentially non-Quisling Poles had nothing in it for him or his plans for a secure buffer zone of satellites.
But say that, perhaps convinced by his generals, perhaps as part of some greater calculation, Stalin orders that the rebels be supported and an invasion of Warsaw be attempted.
What happens?